Solder reflow with microwave energy

ABSTRACT

The present invention includes a mechanical joint between a die and a substrate that is reflowed by microwave energy and a method of forming such a mechanical joint by printing a solder over a substrate, placing the solder in contact with a bump over a die, reflowing the solder with microwave energy, and forming a mechanical joint from the solder and the bump.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of semiconductorintegrated circuit (IC) manufacturing, and more specifically, to amethod of packaging flip chips.

[0003] 2. Discussion of Related Art

[0004] Chip-to-package interconnections have traditionally involvedwirebonding. Wirebonding is the use of very fine metal wires to join thecontacts at the top surface of the chip with the corresponding contactsat the top surface of the substrate. However, as transistor sizescontinue to shrink, chip performance and chip reliability are becominglimited by the chip-to-package interconnections. Consequently,wirebonding is being superseded by solder bumping.

[0005] Solder bumping has many advantages-over wirebonding. First, bumpsmay be placed anywhere over the chip so the input/output (I/O) densityis significantly increased. Second, the bumps reduce the length of theinterconnections so chip performance is greatly enhanced. Third,eliminating the edge-connections associated with wirebonding allows ahigher level of integration of the chip with the packaging, thusdecreasing the footprint of the package.

[0006] As part of the process of solder bumping, a chip on a die isattached to a substrate in a package by reflowing solder in a convectionoven. Reflow involves melting the solder to reach an energetically morefavorable shape and state. However, all components on the die and thesubstrate are heated so damage may result, especially during cool down,from the large stresses that arise at interfaces between differentmaterials. Such thermal mismatch occurs because the Coefficient ofThermal Expansion (CTE) of the substrate may be about ten times largerthan the CTE of the chip. The difference in CTE with the chip is largerfor organic substrates than for ceramic substrates. The large stressesmay lead to delamination and cracks which tend to propagate when thechip is thermally loaded, especially under conditions of high humidity.

[0007] Thus, what is needed is a method of selectively reflowing solderwithout affecting other components on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIGS. 1(a)-(e) is an illustration of an elevation view of anembodiment of a method of reflowing solder according to the presentinvention.

[0009]FIG. 2 is an illustration of an elevation view of an embodiment ofa method of forming a mechanical joint according to the presentinvention.

[0010]FIG. 3 is an illustration of an elevation view of an embodiment ofa mechanical joint between a solder over a substrate and an AlternateBall Metallurgy (ABM) over a die according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0011] In the following description, numerous particular details, suchas specific materials, dimensions, and processes, are set forth in orderto provide a thorough understanding of the present invention. However,one skilled in the art will realize that the invention may be practicedwithout these particular details. In other instances, well-knownsemiconductor equipment and processes have not been described inparticular detail so as to avoid obscuring the present invention.

[0012] The present invention comprises a mechanical joint between asubstrate and a die and a method of using microwave energy to form themechanical joint. Microwave energy may be used to selectively reflowsolder on the substrate to form the mechanical joint with input/output(I/O) connections at the surface of the die or flip chip. The mechanicaljoint in Surface Mount Technology (SMT) allows an electrical connectionfor power, ground, or signal in and out of the die.

[0013] In one embodiment, the substrate may be an interposer in aflip-chip Chip Scale Package (CSP) or a Ball Grid Array (BGA). Inanother embodiment, the substrate may be a printed circuit board (PCB)in Direct Chip Attach (DCA). The substrate may be ceramic, organic, or acomposite. Examples include Alumina, FR-4, and resin. The die may be anactive or passive electronic component. Examples include IntegratedCircuits (IC) chips, such as a microprocessor or a memory chip. Otherexamples include surface mount components, such as capacitors andresistors. The die may include dielectric, semiconducting, and metallicmaterials.

[0014] Various embodiments of the method of the present invention willbe described next. A die 100 has an electronic component 105 that may beactive or passive. An embodiment is shown in FIG. 1(a). In oneembodiment, the electronic component 105 is a microprocessor havingdevices connected with a multilevel interconnect system. The multilevelinterconnect system may include a stack of 2-7 layers of conductinglines connected vertically with conducting plugs. The conducting linesare electrically isolated, vertically and laterally, with dielectricmaterial. The conducting lines may be Aluminum or Copper. The conductingplugs may be Tungsten or Copper. The dielectric material may be SiliconOxide, Silicon Nitride, or Silicon Oxynitride. Other material may beincluded, such as adhesion layers, diffusion barrier layers,anti-reflective coating (ARC) layers, and capping layers.

[0015] An input/output (I/O) connection, such as a bond pad 110, islocated on the surface of the electronic component 105 and covered witha passivation layer 120. An embodiment is shown in FIG. 1(a). The bondpad 110 may be Aluminum. In one embodiment, the passivation layer is aphotoimageable polyimide 120 that is spin-coated.

[0016] The polyimide 120 is exposed and developed to open a via 125 overthe bond pad 110. An embodiment is shown in FIG. 1(b). The polyimide 120is a thermoplastic polymer that, after curing, has very good propertieswith respect to thermal stability (over 400 degrees Centigrade),mechanical toughness (high tensile strength and high elastic modulus),dielectric constant, and chemical resistance.

[0017] After plasma pre-clean to remove oxide from the surface of thebond pad 110, an Under Bump Metallurgy (UBM) 130 base structure issputtered over the polyimide 120 and the portion of the bond pad 110uncovered by the via 125. An embodiment is shown in FIG. 1(c). The UBM130 may include several layers, such as a lower layer 133 and an upperlayer 136.

[0018] The lower layer 133 may be formed from Titanium with a thicknessof about 200-1500 Angstroms. Other possible materials for the lowerlayer 133 include Titanium-Tungsten, Tantalum, or Chromium. The lowerlayer 133 provides good adhesion to the bond pad 110 and the polyimide120 to protect the multilevel interconnect system below the bond pad 110from corrosion.

[0019] The upper layer 136 may be formed from Nickel-Vanadium with athickness of about 1000-8000 Angstroms. Other possible materials for theupper layer 136 of the UBM 130 include Nickel, Copper, Gold,Nickel-Gold, or Copper-Gold. The upper layer 136 provides good adhesionto the lower layer 133 and is wettable by solder 150. The upper layer136 also acts as a diffusion barrier to prevent interdiffusion of metalsbetween the solder 150 and the bond pad 110 that may result inembrittlement, higher resistivity, and premature structural failure.

[0020] The UBM 130 is covered with a thick layer of photoresist 140. Thephotoresist 140 is exposed and developed to create an opening 145 over aportion of the upper layer 136 of the UBM 130. An embodiment is shown inFIG. 1(d). The opening 145 in the photoresist 140 may be locateddirectly over the bond pad 110. In another embodiment, the opening 145may be offset to one side of the bond pad 110.

[0021] The UBM 130 serves as a low-resistance electrical path forelectroplating a solder 150 from a plating solution through the opening145 in the photoresist 140 over the exposed portion of. the upper layer136 of the UBM 130. The solder 150 may be formed from variouscompositions of Lead-Tin. Tin prevents oxidation and strengthens thebonding of the solder 150 to the UBM 130.

[0022] Once the thickness of the solder 150 being selectively depositedthrough the opening 145 in the photoresist 140 exceeds the thickness ofthe photoresist 140, the solder 150 will spread out in a mushroom shape.An embodiment is shown in FIG. 1(d). The thickness of the solder 150, orbump height, needs to be well-controlled, with a standard deviation ofless than 2.5 microns (um) within the die and across the lot. Theminimum distance between the centers of adjacent solder 150, or bumppitch, is usually limited by assembly and reliability considerations andis typically 100.0-250.0 microns (um).

[0023] After electroplating of the solder 150 is completed, thephotoresist 140 is stripped off. Then, a wet etch solution is used toselectively etch the UBM 130 without etching the solder 150. Removal ofthe portions of the upper layer 136 and the lower layer 133 that are notcovered by the solder 150 will electrically isolate the solder 150 fromeach other.

[0024] The next step is to reflow the solder 150 to form a bump 155. Anembodiment is shown in FIG. 1(e). The melting temperature of the solder150 depends on its composition. For example, a high Lead solder, such as95 Pb/5 Sn by weight percent, flows at about 300-360 degrees Centigrade.Upon solidification, the solder draws into an approximately sphericalshape due to surface tension.

[0025] A convection oven may be used to reflow the solder 150. The covergas in the convection oven may include forming gas. Forming gas may havea passive component, such as 90.0% Nitrogen to prevent formation ofoxides, and an active component, such as 10.0% Hydrogen to chemicallyreduce existing oxides.

[0026] Next, the die 100 is attached to the substrate 170. Solder paste160 may be applied to a pad 172 of the substrate 170 in order to attacha bump 155 of the die 100. The solder paste 160 may indude a solderalloy 165, a flux, a solvent, a surfactant, and an antioxidant. Thesolder alloy 165 is a combination of metals that melts or reflows at acertain temperature. The flux creates a wettable surface for the solderalloy 165 by removing oxides and other contaminants from the surface ofthe bump 155 of the die 100. The solvent prevents the flux fromsublimating or polymerizing when the solder paste 160 is heated. Thesurfactant reduces the surface tension at the interface between thesolder paste 160 and the bump 155 to further promote wetting of thesolder alloy 165. The antioxidant prevents reoxidation of the surface ofthe bump 155 after the flux has prepared the surface of the bump 155 forsoldering.

[0027] The solder paste 160 may be printed on the pad 172 of thesubstrate 170 with a stencil printer. A stencil is a metal foil that haslaser-cut or chemically etched apertures that match the array of solderbumps 155 on the die 100. The stencil printer has two squeegees: one fora forward stroke and another for a reverse stroke. During printing,solder paste 160 is rolled in front of a squeegee to fill an aperture inthe stencil. Then, the squeegee moves over the stencil and shears offthe solder paste 160 in the aperture. The pressure generated by thesqueegee injects the solder paste 160 into the aperture and onto thecorresponding pad 172.

[0028] Next, the die 100 and the substrate 170 are aligned, placed incontact, and exposed to microwave energy 180. An embodiment is shown inFIG. 2(a). Microwave energy 180 refers to the portion of theelectromagnetic spectrum from 1.0-40.0 GigaHertz (GHz). Microwave energy180 may be generated from a magnetron 190. A magnetron 190 is a powertube oscillator that converts electrical power directly intoRadio-Frequency (RF) power. A pulsed, low wattage power supply cangenerate about 1.0-1.5 kiloWatt (kW) microwave power from a magnetron190. A Continuous-Wave (CW), high wattage power supply can generateabout 2.5-6.0 kW microwave power from a magnetron 190.

[0029] The microwave energy 180 from a magnetron 190 is transmittedthrough a waveguide 195 coupled to an input aperture towards the die 100and the substrate 170 in a chamber of a batch tool or, alternatively, ona conveyor belt of an in-line tool. The walls around the chamber orconveyor belt are electrically conductive and define a resonant cavitywith fixed dimensions. Emissivity of the walls may have a value such as0.8. The cavity may be tuned to the proper resonant frequency and modeto maximize the amount of microwave energy 180 absorbed.

[0030] Whether an object absorbs or reflects microwave energy depends onthe material and, to a lesser extent, its shape and size. For example,Silicon and Carbon absorb microwave energy 180 and are heated upreadily. In contrast, metals reflect microwave energy 180 and are notheated up directly.

[0031] Good thermal management requires that the microwave energy 180within a large volume be well-mixed. In one embodiment, the frequency ofthe microwave energy 180 can be varied very rapidly. Sweeping of thefrequency prevents standing waves and eliminates arcing damage due tobuild-up of charges in metal or Silicon on the die 100.Variable-frequency microwave energy 180 may be provided in a tool suchas a MicroCure 2100 (batch) or 5100 (in-line) system manufactured byLambda Technologies, Inc., of Morrisville, N.C. The microwave energy 180may be set at 5.8 (+/−1.12) GHz with a power output of about 400.0-750.0Watts.

[0032] In another embodiment, a susceptor 200, capable of linear motionand rotational motion, is used to hold the substrate 170 and further mixthe microwave energy 180. The susceptor 200 is formed of a material thatdoes not absorb microwave energy 180.

[0033] In still another embodiment, a stirrer 210, capable of linear androtational motion, is used to further mix the microwave energy 180. Thestirrer 210 is formed of a material that reflects microwave energy 180.

[0034] In one embodiment, the process includes a ramp up rate of about28.0 degrees Centigrade per second to a peak temperature of about221.0-240.0 degrees Centigrade with a dwell time of 15.0-30.0 seconds atthe peak temperature, followed by an unaided cool down to 100.0 degreesCentigrade. Reflow may be performed in an inert atmosphere, such asNitrogen with a purity level of 10.0-25.0 parts per million (ppm).

[0035] The variable-frequency microwave energy 180 radiation is absorbedby the Silicon material in the die 100, transformed into heat bymolecular excitation, and conducted through the die bump 155 toindirectly heat the solder paste 160 on the pad 172 of the substrate170. The heat activates the flux in the solder paste 160 to remove metaloxides from the surface of the bump 155 on the die 100 to improveadhesion of the solder alloy 165. The heat also melts the solder alloy165 in the solder paste 160. Surface tension causes the solder alloy 165to wet and self-align to the bump 155 on the die 100 and, uponsolidification, to form a mechanical joint 225 between the bump 155 onthe die 100 and the corresponding pad 172 on the substrate 170. Anembodiment of a mechanical joint 225 of the present invention is shownin FIG. 2(b).

[0036] Variable-frequency microwave energy 180 allows the desiredtemperature to be achieved quickly and uniformly so as to reflow thesolder alloy 165 without unnecessarily heating up other portions of thesubstrate 170 assembly and package. The selective heating reduces stressin the substrate 170 assembly and package while allowing fasttemperature ramps. In addition to reflowing solder alloy 165 in asignificantly shorter time, the variable frequency microwave energy 180also offers a wider thermal margin between the die 100 and the substrate170. This reduces the stresses on the bump 155 caused by mismatch inCoefficient of Thermal Expansion (CTE). In addition, this allows theusage of a substrate with the bottom side pre-pinned with similar solderpaste as the top side. Otherwise, the chip join process will result inunacceptable True Position Radius (TPR) of the pins or unacceptable pinpull strength.

[0037] The solder alloy 165 may be an eutectic solder, such as 37 Pb/63Sn by weight percent, that reflows at about 180-240 degrees Centigrade.Alternatively, “no-lead” solder, which is more environmentally friendly,may be printed on the substrate 170. For example, a binary alloy of 96.5Sn/3.5 Ag by weight percent that flows at about 230 degrees Centigrademay be used. A ternary alloy of 95.5 Sn/4.0 Cu/0.5 Ag by weight percentthat flows at about 215 degrees Centigrade may also be used.

[0038] In another embodiment, Alternate Ball metallurgy (ABM) 157,rather than Lead-Tin solder 150, is electroplated from solution onto theUBM 130 of the die 100. In one embodiment, the ABM 157 is Copper that iselectroplated with a column or pillar shape onto the UBM 130. In theembodiment using ABM 157, reflow is not done for the ABM 157 so thecolumnar or pillar shape is preserved. FIG. 3 shows an embodiment of amechanical joint 227 of the present invention resulting from reflow ofsolder alloy 165 on a pad 172 of a substrate 170 to attach the ABM 157of the die 100.

[0039] After reflow is completed, residue around the bump 155 or ABM 157may be removed by deflux, such as with Deionized (DI) water. Next,underfill may be dispensed between the die 100 and the substrate 170 andcured to reduce stress on the mechanical joint 225 or 227. Then ashingmay be done, followed by dispensing of epoxy and attachment of a thermalspreader or heat sink.

[0040] Other embodiments, besides electroplating, may be used to form abump of the die 100. Examples include solder stencil printing, solderscreen printing, stud bumping, evaporation through a Molybdenum shadowmask (Controlled Collapse Chip Connection or C4), sputtering,electroless bumping, solder jetting, and polymer bumping.

[0041] Other embodiments, besides stencil printing, may also be used toform the solder on the pad 172 of the substrate 170. Examples of fluxtransfer include pin transfer, screen printing, and direct dipping.Examples of solder ball transfer include gravity-with-stencil andvacuum-and-pick-and-place.

[0042] Many alternative embodiments and numerous particular details havebeen set forth above in order to provide a thorough understanding of thepresent invention. One skilled in the art will appreciate that many ofthe features in one embodiment are equally applicable to otherembodiments. One skilled in the art will also appreciate the ability tomake various equivalent substitutions for those specific materials,processes, dimensions, concentrations, etc. described herein. It is tobe understood that the detailed description of the present inventionshould be taken as illustrative and not limiting, wherein the scope ofthe present invention should be determined by the claims that follow.

[0043] Thus, we have described a method of selectively reflowing solderto form a mechanical joint between a substrate and a die.

We claim:
 1. A method comprising: forming a solder over a substrate;placing said solder in contact with an input/output connection of a die;heating said substrate and said die with microwave energy to reflow saidsolder; and forming a mechanical joint between said substrate and saiddie.
 2. The method of claim 1 wherein frequency of said microwave energyis swept very rapidly.
 3. The method of claim 1 further comprisingmixing said microwave energy by moving said substrate and said dieduring said heating.
 4. The method of claim 1 further comprising mixingsaid microwave energy by stirring with a reflective material.
 5. Themethod of claim 1 further comprising activating flux prior to saidreflowing of said solder.
 6. The method of claim 1 wherein saidinput/output connection is a bump.
 7. The method of claim 6 wherein saidsolder comprises an eutectic solder and said bump comprises a high Leadsolder.
 8. The method of claim 6 wherein said solder comprises a no-Leadsolder and said bump comprises Alternate Ball Metallurgy (ABM).
 9. Amethod comprising: forming a final passivation layer over a die;removing said final passivation layer over a bond pad of said die;forming an Under Bump Metallurgy (UBM) over said bond pad and saidpassivation layer; forming a photoresist over said UBM; uncovering afirst portion of said UBM by selectively removing said photoresist, saidfirst portion disposed over said bond pad; covering said first portionof said UBM by selectively forming a first solder; removing rest of saidphotoresist; removing a second portion of said UBM, said second portionnot covered by said first solder; reflowing said first solder into abump; placing said bump in contact with a second solder on a substrate;and reflowing said second solder with microwave energy.
 10. The methodof claim 9 wherein said microwave energy has variable frequency.
 11. Themethod of claim 9 wherein said reflowing of said first solder into saidbump is with microwave energy.
 12. The method of claim 11 wherein saidmicrowave energy has variable frequency.
 13. The method of claim 9wherein said reflowing of said second solder is at a temperature that isabout 50 degrees Centigrade lower than said reflowing of said firstsolder.
 14. A mechanical joint comprising: a bump on a die; and a solderon a substrate wherein said solder was reflowed by microwave energy. 15.The mechanical joint of claim 14 wherein said microwave energy hasvariable frequency.
 16. The mechanical joint of claim 14 wherein saidbump was reflowed by microwave energy.
 17. The mechanical joint of claim16 wherein said microwave energy has variable frequency.